Commonly-assigned U.S. Pat. Nos. 4,378,893 and 4,407,422 disclose composite plastic closure constructions which have proven to be commercially successful. In particular, the closure constructions disclosed in these patents have proven highly effective for use on containers having carbonated or otherwise pressurized contents, with the closure constructions further lending themselves to very economical, high-speed manufacture. Commonly-assigned U.S Pat. Nos. 4,343,754 and 4,497,765 disclose methods and apparatus for effecting efficient manufacture of the above types of closures.
In accordance with the forming methods disclosed in the above patents, an outer plastic shell or cap of the closure is formed by molding, preferably compression molding, wherein plastic material is molded between a generally cup-shaped female molding cavity, and a male molding plunger which enters the female cavity. In order to permit use of the plastic closures on containers having a thread formation, an annular skirt portion of the plastic cap is formed with internal threads, which such internal threads are formed by external thread-forming grooves defined by the male molding plunger.
In accordance with the teachings of the above patents, high-speed closure manufacture is promoted by "stripping" or "popping-off" a molded plastic cap from the male molding plunger (as opposed to relatively rotating the closure so that it is "unthreaded" from the male plunger). As will be appreciated, this stripping action results in outward deformation of the closure's skirt portion as the molded closure threads are urged out of the thread-forming grooves of the male plunger as the molded cap is removed from the plunger. Relative motion of the male plunger with respect to a stripper sleeve positioned coaxially thereabout effects stripping of the molded cap in this manner after the cap has been removed from the associated female molding cavity.
Economical closure manufacture is in part greatly dependent upon the speed with which cap molding can be achieved. In this regard, the nature of the above-described "stripping" step has been found to act to limit the speeds at which the molding equipment can be operated. More specifically, it has been found that as forming speeds are increased, the stripping of the molded cap from the male molding plunger can result in unacceptable deformation of the just-formed closure threads.
Such thread deformation is believed to be a result of the relatively high stresses exerted on the closure thread during stripping (bearing in mind that sufficient force must be generated to flex the closure skirt portion outwardly for stripping), with the thread comprising one of the thickest cross-sectional portions of the closure. Since the forming "cycle time" for manufacture of closures of the present type is only a matter of seconds, the relatively great mass of plastic material at the closure thread may not be sufficiently cooled and cured so as to provide the threads with the requisite strength for high-speed stripping. Thus, relalively increased forming speeds can result in the closure thread being unacceptably deformed or "wiped-off" as the closure is stripped from the forming plunger.
Since increased forming speeds promote economical closure manufacture, it is highly desirable to provide a threaded plastic closure construction configured to promote high-speed manufacture. Additionally, it is important to bear in mind that any manner by which the amount of plastic material employed in a closure can be reduced, without impairing the closure's strength and sealing characteristics, is very desirable.